A conventional counter-rotating axial flow fan is disclosed in Japanese Patent No. 4128194. The counter-rotating axial flow fan includes a casing including an air channel having a suction port on one side in an axial direction and a discharge port on the other side in the axial direction, a front impeller including a plurality of front blades and configured to rotate in the air channel, a rear impeller including a plurality of rear blades and configured to rotate in the air channel, and a middle stationary portion formed by a plurality of stationary blades or struts disposed to be stationary between the front impeller and the rear impeller in the air channel.
In the counter-rotating axial flow fan, the front impeller, the rear impeller, and the middle stationary portion are elaborately shaped to reduce noise. However, thorough studies on the relationship between the middle stationary portion and noise have not been made so far.